1. Field of the Invention
The present invention relates to the preparation of oxides, and, more particularly, to the preparation of crystallized powder of a substantially single size. Although the invention specifically contemplates use of the oxides as phosphors, because particles of uniform size pack more efficiently, the oxides prepared in accordance with the invention may also be used in the fabrication of dense, sintered ceramics, such as ferrite cores (e.g., yttrium iron garnet), transducers (barium titanate), and the like.
2. Description of Related Art
Powders of phosphors are used for display purposes. The typical particle size of the powder ranges from about 2 to 10 .mu.m, depending on the energy of the electron beam, although larger particle sizes may be employed as the energy of the electron beam is increased. The particle size distribution itself is typically about 4 .mu.m.
The more uniform the particle size of a phosphor, that is, where the particle size distribution approaches zero, the better the resolution of the phosphor display. Current approaches to obtaining phosphor particles of a substantially single particle size are based on the control of particle-size distribution of the crystallized powder during the synthesis of a multicomponent system, or compound. Unfortunately, the temperature required for synthesis is above the temperature for particle growth and sintering. Consequently, there is either no control of particle size or no control of particle size distribution, which, at best, is broad.
The next alternative is sizing, such as by sedimentation in a neutral liquid or elutriation in air, to obtain a sharper particle size distribution. However, if a sharper size distribution and/or a smaller peak size is desired, the yield is poor.
When particle size distribution, concomitant with synthesis, cannot be realized, the product is comminuted, usually by ball milling, to obtain a working particle size distribution and may be sold as such without further sizing. This provides an inferior starting material, because of considerable surface damage of the particles, in addition to impurity pickup during grinding.
The amorphous monosize preparation of a one-component oxide system, such as SiO.sub.2, has been achieved. However, the process for its conversion to a monosize crystal powder is not obvious. Thus far, such a demonstration has not been achieved. Mere heating to crystallize the monosize amorphous form is plagued by competition from two other processes: (1) transformation into the vitreous state, thereby precluding crystallization, and (2) sintering, which obliterates the monosize condition. The problem is complicated further in a multi-component system where the desired end-product is a compound of definite (fixed) stoichiometry, as in yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12 ; YAG), yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12 ; YIG), zircon (ZrSiO.sub.4), and the like.
Thus, there remains a need to provide a process for the preparation of oxides, whether single component or multi-component, of substantially single particle size, in which the particle size distribution is essentially zero.